Process and composition for treating metal surfaces using trivalent chromium compounds

ABSTRACT

Aqueous compositions useful as pretreatments prior to painting to reduce the formation of rust in the uncoated condition. The compositions consist essentially of water, a trivalent chromium compound with the formula Cr(HxPO4)3, where x can be 1.5 or 2, a silica compound, and optionally hydrogen peroxide. The composition may have a pH between about 1 and about 4. A process for treating a metal surface includes contacting the surface with such aqueous compositions. The compositions and the processes provide benefits in comparison to the zinc phosphate metal pretreatment thought to be the standard in the industry.

RELATED APPLICATION

This application claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 62/578,787, filed on Oct. 30, 2017, thecontents of which are incorporated in this application by reference.

TECHNICAL FIELD

This invention relates generally to compositions and the use of suchcompositions for passivating and improving the paint adhesion of metalsurfaces. More particularly, this invention relates to an aqueouscomposition, suitable for use as a dried-in-place coating for metal andto methods for using that composition.

BACKGROUND OF THE INVENTION

Known methods of treating metal surfaces to improve paint adhesion andcorrosion resistance of painted metal surfaces include two generalclasses of chemistries. The first class is based on traditionalconversion coating types of chemistries, such as zinc phosphate, ironphosphate, chromium chromate, chromium phosphate, etc. The second classis based on more recent developments in the metal pretreatment industryand is characterized by what is now referred to as “dried-in-place”technology. Traditional conversion coating chemistries require rinsingof the metal substrate to remove applied pretreatment solution.Dried-in-place chemistries allow for the applied solutions to be driedon the metal substrate to which they are applied, without rinsing priorto the application of a paint.

With regard to dried-in-place processes, it is generally known to treatthe surfaces of metals, such as iron, zinc, cadmium, aluminum, or alloysthereof with aqueous hexavalent chromium solutions which containchemicals that dissolve the surface of the metal and form insolublefilms known as “chromate conversion coatings.” These coatings, whichcontain hexavalent chromium, are corrosion resistant and protect themetal from various elements which cause corrosion. In addition, it isknown that hexavalent chromate conversion coatings generally have goodpaint bonding characteristics and, therefore, provide an excellent basefor paint or other finishes.

Although the aforementioned coatings enhance corrosion resistance andpaint bonding properties, the coatings have a serious drawback.Hexavalent chromium shows toxicological effects and has been determinedby the U.S. Environmental Protection Agency as a risk to the environmentand by the U.S. Occupational Safety and Health. Agency as a health risk.Moreover, chemistries based on hexavalent chromium are classified ascarcinogens by these agencies.

Within the past few decades, various compositions and processes, notrelying on hexavalent chromium, have been described and used fortreating metal surfaces. One such example is described in U.S. Pat. No.4,169,741 to Rausch et al., which describes a dried-in-place methodusing a composition comprising, among other elements, trivalentchromium, phosphate ion, and dispersed silica.

It is highly desirable to provide coatings and processes which are freeof hexavalent chromium, but still capable of improving paint adhesionand corrosion resistance of metal surfaces, such as aluminum, which arecomparable to conventional hexavalent chromium-based coatings.Additionally, there is a need to provide protective coatings havingexcellent corrosion resistance with lowered resistivities and adequatecoating weights.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a composition for treating a metalsurface to improve paint adhesion and corrosion resistance and/ormaintain low electrical contact resistance. The composition may be usedas a pre-paint treatment and is intended to be used to treat a range ofmetals including copper, brass, magnesium, aluminum, iron, zinc,cadmium, or alloys thereof. In one embodiment, the composition compriseswater, a trivalent chromium compound with the formula Cr(H_(x)PO₄)₃,where x can be 1.5 or 2, a silica compound, and optionally hydrogenperoxide.

In another embodiment, the composition consists essentially of water, atrivalent chromium compound with the formula Cr(H_(x)PO₄)₃, where x canbe 1.5 or 2, a silica compound, and hydrogen peroxide.

In another embodiment, the composition consists of water, a trivalentchromium compound with the formula Cr(H_(x)PO₄)₃, where x can be 1.5 or2, a silica compound, and hydrogen peroxide.

In an embodiment, the trivalent chromium compound is present in anamount of between: about 10% wt. % to about 5% wt. %, about 10% wt. % toabout 2.5% wt. %, about 10% wt. % to about 1% wt. %, or about 10% wt. %to about 0.5% wt. %.

In another embodiment of the invention, the silica is present in anamount of less than 2% wt. %; less than 1.75% wt. %; or less than 1.5%wt. %.

In still another embodiment, the present invention is a process fortreating a metal surface. The process includes the step of contactingthe metal surface with a composition comprising water, a trivalentchromium compound with the formula Cr(H_(x)PO₄)₃, where x can be 1.5 or2, a silica compound, and optionally hydrogen peroxide.

In a further embodiment, the invention is a process for treating a metalsurface comprising the steps of: (1) cleaning the metal surface to forma cleaned metal surface; (2) rinsing the cleaned metal surface withwater to form a rinsed metal surface; and (3) contacting the rinsedmetal surface with a composition comprising water, a trivalent chromiumcompound with the formula Cr(H_(x)PO₄)₃, where x can be 1.5 or 2, asilica compound, and hydrogen peroxide.

In a still further embodiment, the process additionally comprises, aftercontacting the rinsed metal surface with the composition, again rinsingthe metal surface with water and then sealing the metal surface.

The processes may additionally comprise, before the first contactingstep, the step of cleaning the metal surface with an alkaline cleanerand rinsing. The processes may further comprise, after contacting themetal surface with the pretreatment composition, the steps of rinsingthe metal surface with water and then painting the surface of the metal.The pH of the aqueous pretreatment composition comprising water, atrivalent chromium compound with the formula Cr(H_(x)PO₄)₃, where x canbe 1.5 or 2, a silica compound, and hydrogen peroxide, is preferablyacidic and more preferably has a pH of between about 1 and about 4.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention is best understood from the following detailed descriptionwhen read in conjunction with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the various features are arbitrarilyexpanded or reduced for clarity. Included in the drawing are thefollowing figures:

FIG. 1 is a side-by-side photograhic comparison of the results fromComposition 1 as compared to the Control composition for panels scribedand exposed to neutral salt spray for a period of 250 hours per ASTMB117;

FIG. 2 is a side-by-side photographic comparison of the results fromComposition 1 as compared to the Control composition for panels scribedand exposed to neutral salt spray for a period of 250 hours per ASTMB117;

FIG. 3 is a side-by-side photographic comparison of the results fromComposition 1 and Composition 2 as compared to the Control compositionfor panels scribed and exposed to neutral salt spray for a period of 500hours per ASTM B117;

FIG. 4 is a side-by-side photograph of the results from. Composition 2for panels scribed and exposed to neutral salt spray for a period of 250hours per ASTM B117;

FIG. 5 is a side-by-side photographic comparison of the results fromComposition 1 as compared to the Example 1 composition disclosed in theRausch patent for panels scribed and exposed to neutral salt spray for aperiod of 250 hours per ASTM B117; and

FIG. 6 is a side-by-side photographic comparison of the results fromComposition 1 as compared to the Example 1 composition disclosed in theRausch patent for panels scribed and exposed to neutral salt spray for aperiod of 250 hours per ASTM B117.

DESCRIPTION OF THE INVENTION

The present invention is directed to compositions and processes fortreating a metal surface. Compositions according to the presentinvention comprise, preferably consist essentially of or most preferablyconsist of water, a trivalent chromium compound with the formulaCr(H_(x)PO₄)₃, where x can be 1.5 or 2, a silica compound, andoptionally hydrogen peroxide. Processes according to the presentinvention include contacting a metal surface with the above composition,including: water, a trivalent chromium compound with the formulaCr(H_(x)PO₄)₃, where x can be 1.5 or 2, a silica compound, andoptionally hydrogen peroxide.

Aqueous compositions of the present invention are applied to a metalsurface after cleaning but before some final coat is applied to themetal surface, contributing to at least one of the following: (1)improving the paint adhesion of the metal surface; (2) improving thecorrosion resistance of the metal surface; and (3) maintaining orreducing the resistivity of the metal surface. Compositions of thepresent invention include compositions which significantly improve oneor two of these characteristics, even though at least one of the othersis worsened to a lesser extent. The improvement could be due to thecompositions alone or the compositions in combination with other processsteps. Such compositions are referred to herein or in the metaltreatment industry as pretreatment compositions, conversion coatings, orworking compositions. “Resistivity” is defined as resistance per unitsurface area; typical units of resistivity are microhms per square inch.

As used herein, the term “hexavalent chromium compound” means compounds,namely salts, of chromium in which the chromium has a valence of plus 6.A wide range of anions could be used, and more than one hexavalentchromium compound could be used. Preferably, the hexavalent chromiumcompound is anhydrous chromic acid (CrO₃), chromic acid (H₂CrO₄), orchromium chromate (Cr₅O₁₂).

As used herein, the term “metal,” used for example in the phrase “metalsurface,” includes copper, brass, magnesium, aluminum, iron, zinc,cadmium, or alloys thereof. Each metal listed includes both theelemental metal and alloys thereof; for example, the term “aluminum”means aluminum and aluminum alloys. The term “alloy” is a metal in whichthe primary element has a higher content than every other element or acontent equal to the highest content of every other element (e.g., analuminum alloy being a metal in which aluminum is present in an amountat least equal to that of any other element). Iron alloys include coldrolled steel, electro-galvanized steel, and hot-dipped galvanized steel.Preferably, compositions of the present invention are used to treat arange of metals including alloys of copper, brass, magnesium, aluminum,and iron.

As used herein, the term “pretreatment composition” means anycomposition which improves the paint adhesion, lowers electrical contactresistance, and improves corrosion resistance of a metal surface.Aqueous pretreatment compositions are used as a pretreatment beforepainting and may be used as a passivation treatment to reduce theformation of corrosion in the uncoated (unpainted) condition. Thus,although the composition may be called a pretreatment composition forconvenience, it is a composition used to improve the adhesion ofsubsequently applied paint and resist corrosion of the unpaintedsurface.

As used herein, the term “treating” means applying a treatment orcleaning, rinsing, and applying a pretreatment. The pretreatment alsofunctions as a sealant to seal the metal surface, so the term “treating”shall optionally include the step of sealing the metal surface. Further,“treating” optionally can include process steps up through and includingpainting. For example, treatment steps may also include a step ofapplying a decorative coating, such as painting. After applying thepretreatment, the pretreatment may be rinsed first or dried-in-placebefore application of the paint. Each of these steps play a role in afinal product's ability to resist corrosion and minimize paint loss. Asmentioned above, the treatment composition can be used as a pre-painttreatment.

As used herein, the term “trivalent chromium compound” means compounds,namely salts, of chromium in which the chromium has a valence of plus 3.No hexavalent chromium (or at worst a de minimus, inconsequential amountof it) is present in such compounds. A wide range of anions could beused, and more than one trivalent chromium compound could be used.Preferably, the trivalent chromium compound is a compound with theformula Cr(H_(x)PO₄)₃, where x can be 1.5 or 2.

In an alternative embodiment, the composition of the present inventionconsists essentially of water, a compound with the formulaCr(H_(x)PO₄)₃, where x can be 1.5 or 2, a silica compound, andoptionally hydrogen peroxide. While such a composition could includeadditives, it excludes halide ions which promote undercutting. Ifundercutting occurs, reduced performance could follow.

Appropriate concentration ranges of the various components are dependentupon their solubilities. At concentrations too low, there areinsufficient amounts of the constituents to cover the metal surface andperform their functions. Above the solubility limits, the solute maybegin to come out of the solution. In an embodiment of the invention,the trivalent chromium compound is present in an amount of between:about 10% wt. % to about 5% wt. %, about 10% wt. % to about 2.5% wt. %,about 10% wt. % to about 1% wt. %, or about 10% wt. % to about 0.5% wt.%. In an embodiment of the invention, the silica is present in an amountof less than 2% wt. %; less than 1.75% wt. %; or less than 1.5% wt. %.

In an embodiment of the invention in which the precursor hexavalentchromium compound is anhydrous chromic acid (CrO₃), the chromic acid isfirst reduced in the presence of phosphoric acid to create a trivalentchromium compound: chromium dihydrogen phosphate (Cr(H₂PO₄)₃). Hydrogenperoxide is the reducing agent. After the chromium is reduced, silica isadded. The following pH range has been found to be preferred givencertain other conditions: 1 to 4.

The compositions given above are of the working bath. It is desirable,of course, to transport the product in the form of a concentrate, namelyup to a 10 to 100 fold increase in concentration of the above workingbath concentrations. The upper limit of such concentrates is thesolubility limit of the first constituent to meet or exceed itssolubility limit.

The pH of the compositions is preferably, when the composition is usedto treat cold rolled steel, between 1 to 4. More preferably, the pH isabout 1.75.

Compositions according to the invention may be made by mixing theingredients in any of a number of sequences. In a non-limitingembodiment where a hexavalent chromium compound is a precursor material,the hexavalent chromium compound is first reduced in the presence ofphosphoric acid to create a trivalent chromium compound: chromiumdihydrogen phosphate (Cr(H₂PO₄)₃). Hydrogen peroxide is the reducingagent. After the chromium is reduced, silica is added. In anothernon-limiting embodiment where a hexavalent chromium compound is aprecursor material, the hexavalent chromium compound is metered into asolution containing phosphoric acid and hydrogen peroxide until thehexavalent chromium compound is fully reduced. After the chromium isreduced, silica is added.

In one non-limiting embodiment, a concentrate (i.e., master batch) iscreated by combining the trivalent chromium compound with the formulaCr(H_(x)PO₄)₃, where x is between 1.5 and 2, with water. A silicacompound can then be added to the solution. The concentrate can then bediluted, preferably with deionized water, to create the desiredconcentration at the metal treatment site prior to use.

In a non-limiting embodiment, upon completion of the reaction someresidual hydrogen peroxide may remain. Conversely, in anothernon-limiting embodiment, the hydrogen peroxide is fully utilized.Preferably, upon completion of the reaction the hydrogen peroxideconcentration is within the following range: from 0 to about 0.85 gramsH₂O₂ (35%) per gram of Cr(H₂PO₄)₃.

Additional components that are well-known in the art could be includedin compositions of the present invention. For example, wetting agents,such as fluorosurfactants, may be included to improve wetting. In somecases, thickeners might also be included if an application requires ahigher viscosity.

In a process of the present invention, a metal surface is coated with apretreatment composition of the present invention. In this coating step,the composition may contact the metal surface by any number oftechniques known in the art. One such method is immersion coating inwhich the metal is immersed in the bath of pretreatment. Othertechniques known in the art including spraying, roll coating, or reverseroll coating, as well as manual application (e.g., brushing). Thecoating step is done for a time sufficient to achieve the desiredcoating weight on the metal surface, which can be determinedempirically. This desired coating weight will depend on a number offactors well-known in the art. In one embodiment, the amount of coatingis sufficient to leave from about 0.1 to about 30 milligrams of chromiumand phosphate per each square foot of dried metal surface as determinedby the weight-strip-weight method. In another embodiment, the amount ofcoating is sufficient to leave from about 1 to about 10 milligrams ofchromium per each square foot of dried metal surface as determined byx-ray fluorescence, and most preferably about 2.5 to about 3.5milligrams of chromium per each square foot of dried metal surface asdetermined by x-ray fluorescence. By using a solution with a higherconcentration of the included elements, it is possible to leave thedesired amount of the dried coating with less residence time.

A process for treating a metal surface to improve corrosion resistance,improve paint adhesion, and/or maintain low electrical contactresistance comprises: (1) cleaning the metal surface to form a cleanedmetal surface; (2) rinsing the cleaned metal surface with water to forma rinsed metal surface; and (3) contacting the rinsed metal surface witha composition comprising water, a trivalent chromium compound with theformula Cr(H_(x)PO₄)₃, where x can be 1.5 or 2, a silica compound, andhydrogen peroxide.

The cleaning step may be carried out in any manner known in the art. Thetypes of cleaners suitable for use in the present invention will varywith a number of factors, including the metal being treated, the desiredapplication, and the amount and type of impurities and soils on themetal surface. As such, the preferred cleaners can be determinedempirically based on these factors. In one non-limiting embodiment, analkaline cleaner is used. An exemplary alkaline cleaning agent which canbe used in connection with the present invention is Bulk Kleen® 841MCcleaner, an alkaline liquid cleaner sold by Bulk Chemicals, Incorporatedof Reading, Pa. In another non-limiting embodiment, a phosphoric acidcleaner is used. In general, the cleaning step may be accomplished bycontacting the metal surface with a bath of an alkaline cleaningsolution to form a cleaned metal surface. The alkaline cleaning solutionmay be an aqueous solution of an alkaline cleaning agent. The cleaningbath cleans the metal surface by removing oil and other contaminantsfrom the metal surface. The cleaning bath is effective to remove theloose impurities and surface soils. Thus, the cleaning bath removessoils and certain impurities from the metal surface. If the metalsurface is heavily soiled, a detergent cleaner additive may be includedin the cleaning step.

A metal surface which has been contacted by an alkaline cleaningsolution is referred to herein as a “cleaned metal surface.” It iscleaned in the sense that it has been exposed to a cleaning bath. It maynot be completely cleaned, however, in the sense that substantially allof the impurities have been removed such that it is ready to be exposedto a pretreatment composition. In some cases, it may be adequatelycleaned, but in other cases, it should first be rinsed with water beforebeing contacted with a pretreatment composition (i.e., substantially allof the impurities are removed by that point).

The rinsing step is well-known in the art, and deionized water ispreferably used. The use of deionized water avoids the introduction ofany deleterious ions, such as chloride ions, into the system. Therinsing step can be two-fold, with a first rinsing step done using tapwater and then rinsing with deionized water.

After step (3) above, contacting the metal surface with the composition,the metal surface may be rinsed with water once again, as is well-knownin the art. The rinsed metal surface can then be sealed. Any chemicalsealing composition well-known in the industry can be used. In apreferred embodiment, the pretreatment composition includes just acomposition comprising water, a trivalent chromium compound with theformula Cr(H_(x)PO₄)₃, where x can be 1.5 or 2, a silica compound, andhydrogen peroxide. When a sealing composition is used, an interveningrinsing step is preferably applied.

After step (3) or any subsequent sealing step, the metal surface may bedried, or rinsed and dried, and then a decorative coating may be appliedto it. For example, the metal surface may be painted or lacquered, orfirst primed and then painted. Such steps, priming and painting, areknown in the art as “finishing steps,” and any known and suitablefinishing steps may be used. Suitable paints include acrylic paints andfluorocarbon paints, among others.

As can be inferred, after step (3) above or any subsequent sealing step,the metal surface can be dried and then a decorative coating (a paintlayer) is applied, without an intervening rinsing step between thesesteps. This alternative process is known as a “dried-in-place”pretreatment. Regardless of whether the pretreatment is “dried-in-place”or there is an intermediate rinsing step, any known method of drying maybe employed. The coating may be dried by, for example, using an oven,forced air, etc.

Determining the times of treatments of the metal surfaces with the bathsof the various steps is well-known in the art. They need only be longenough to permit a sufficient time for cleaning (in the case of thecleaning step) or reaction (in the case of the pretreatment or sealingsteps). They can be very short or as long as thirty minutes and dependon the stage of treatment, the type of application (e.g., immersion,spray), the type of metal surface, and the desired coating weight, amongother factors. The immersion time of a substrate into the compositionsolution will vary with the stage, and generally varies betweenapproximately 1 minute up to about 10 minutes. The times for immersionare typically longer than when spray is used as the method of contact.Rinse times in general can be fairly short, e.g., 30 seconds to oneminute. The specific times of treatment may vary over wide ranges andcan be readily determined by one of ordinary skill in the art.

The desirable performance characteristics of the present invention canbe achieved by the processing steps described above to produce apretreated metal surface with good paint adhesion and corrosionresistance. These characteristics are obtained on the metal surfacewithout a decorative coating. Accordingly, the treated metal surface canbe used as unpainted products and will exhibit corrosion resistance evenif there is a delay between the treatment steps and any subsequentpainting.

The compositions and processes of the present invention provide thestated benefits without the use of additional components which affectthe basic and novel characteristics of the invention. When added to thecomposition in sufficient amounts, other components may affect the novelcharacteristics. For example, certain components may make thecompositions unstable. Such components may affect the shelf-life of thetreatment. Other components may degrade the performance of thecompositions and processes of the present invention.

In summary, the present invention provides environmentally friendlycompositions and processes for treating metal, while still maintainingexcellent paint adhesion and corrosion resistance. More particularly,the present invention avoids the use of hexavalent chromium, and itsassociated health hazards and disposal problems.

EXAMPLES

The following examples are included to more clearly demonstrate theoverall nature of the invention. FIGS. 1-6 illustrate the improvedresults obtained by employing aqueous compositions of this invention.These examples are exemplary, not restrictive, of the invention.

In the below experiments, a batch of Cr(H₂PO₄)₃ was first synthesizedvia the combination of the raw materials identified in Table 1 below.

TABLE 1 Raw Material Mass (g) Wt. % CrO₃ (40%) 63.5 29.1 H₃PO₄ (75%)99.5 45.6 H₂O₂ (35%) 55.1 25.3

A total of 87.1 grams of chromium dihydrogen phosphate (Cr(H₂PO₄)₃) wassynthesized. Additional water was added to the solution and the finalconcentration of the master batch was determined to be 39.2% wt. %. Thismaster batch was further diluted with water to create two separatecompositions identified in. Table 2 below.

TABLE 2 Composition 1 Composition 2 (Identified as #4 (Identified as #6in the Figures) in the Figures) Raw Material Wt. % Active % Wt. % Active% Cr(H₂PO₄)₃ (39.2%) 22.96 9.00 15.31 6.00 Water 63.84 89.42 75.89 92.94Aerodisp W7512S (12%) 13.20 1.58 8.80 1.06

Preparation of Test Panels—Composition 1

Cold rolled steel panels were treated via the following spray process.First, the panels were cleaned with a 3% v/v solution of BulkKleen®841MC for 5 seconds at 165° F. Bulk Kleen®841MC is an alkalinecleaner designed to clean steel, zinc, and aluminum parts. Second, thepanels were rinsed at ambient temperature for 30 seconds. Third, thepanels were again cleaned with Bulk Kleen®841MC at 3% v/v for 5 secondsat 165° F. Fourth, the panels were rinsed with deionized water atambient temperature for 30 seconds. Fifth, Composition 1 was applied tothe panels using a roll coater. Sixth, the panels were dried using hotair. Seventh, all panels were painted using a high reflectance white,single coat polyester at a dry film thickness of 0.7-0.8 mils.

Preparation of Test Panels—Composition 2

Cold rolled steel panels were treated via the following spray process.First, the panels were cleaned with a 3% v/v solution of BulkKleen®841MC for 5 seconds at 165° F. Second, the panels were rinsed atambient temperature for 30 seconds. Third, the panels were again cleanedwith Bulk Kleen®841MC at 3% v/v for 5 seconds at 165° F. Fourth, thepanels were rinsed with deionized water at ambient temperature for 30seconds. Fifth, Composition 2 was applied to the panels using a rollcoater. Sixth, the panels were dried using hot air. Seventh, all panelswere painted using a high reflectance white, single coat polyester at adry film thickness of 0.7-0.8 mils.

Preparation of Test Panels—Control Formula

Cold rolled steel panels were treated via the following spray process.First, the panels were cleaned with a 3% v/v solution of BulkKleen®841MC for 5 seconds at 165° F. Second, the panels were rinsed atambient temperature for 30 seconds. Third, the panels were again cleanedwith Bulk Kleen®841MC at 3% v/v for 5 seconds at 165° F. Fourth, thepanels were rinsed with deionized water at ambient temperature for 30seconds. Fifth, Bulk Bond®780 12% v/v was applied at 165° F. in a spraywasher for 8 seconds. Bulk Bond®780 is a liquid chemical used in coilcoating processes to produce a corrosion-resisting, paint-bonding, ironphosphate coating on steel. Afterwards the panels were rinsed withdeionized water. Sixth, the panels were dried using hot air. Seventh,the control panels were then sealed with Bulk Bond®NP250 35% v/v using aroll coater. Bulk Bond®NP250 is a liquid chrome/silica product designedto be mill applied as a passivation/pretreatment for multi-metalsurfaces. Eighth, all panels were dried and painted using a highreflectance white, single coat polyester at a dry film thickness of0.7-0.8 mils.

Preparation of Test Panels—Rausch Formula

Cold rolled steel panels were treated via the following spray process.First, the panels were cleaned with a 3% v/v solution of BulkKleen®841MC for 5 seconds at 165° F. Second, the panels were rinsed atambient temperature for 30 seconds. Third, the panels were again cleanedwith Bulk Kleen®841MC at 3% v/v for 5 seconds at 165° F. Fourth, thepanels were rinsed with deionized water at ambient temperature for 30seconds. Fifth, the composition outlined in Example 1 of the Rausch etal. patent was applied to the panels using a roll coater. Sixth, thepanels were dried using hot air. Seventh, all panels were painted usinga high reflectance white, single coat polyester at a dry film thicknessof 0.7-0.8 mils.

All test panels were then exposed to neutral salt spray for a period of250 or 500 hours per ASTM B117. Photographic comparisons of the resultsobtained are displayed in FIGS. 1-6. As can be seen, Composition 1. andComposition 2 provided comparable protection to the control formula(i.e., Bulk Bond®780) and outperformed the Rausch et al. formula.Specifically, Composition 1 and Composition 2 exhibited similarcorrosion and pitting as the control formula, whereas the Rausch et al.formula exhibited increased corrosion and pitting.

Without being bound by any specific theory, it is believed thatComposition 1 and Composition 2 outperformed the Rausch et al. formulabecause of the additional ions other than chromium, phosphate, andsilica present in the Rausch et al. formula.

Although illustrated and described above with reference to certainspecific embodiments and examples, the present invention is neverthelessnot intended to be limited to the details shown. Rather, variousmodifications may be made in the details within the scope and range ofequivalents of the claims and without departing from the spirit of theinvention. It is expressly intended, for example, that all rangesbroadly recited in this document include within their scope all narrowerranges which fall within the broader ranges. It is also expresslyintended that the steps of the methods of using the various compositionsdisclosed above are not restricted to any particular order.

What is claimed is:
 1. An aqueous pretreatment composition for treatinga metal surface, the composition comprising: water; silica; a trivalentchromium compound with the formula Cr(H_(x)PO₄)₃, where x can be 1.5 or2.
 2. The aqueous pretreatment composition of claim 1, wherein thepretreatment composition is acidic.
 3. The aqueous pretreatmentcomposition of claim 1, wherein the pretreatment composition has a pH ofbetween about 1 and about
 4. 4. The aqueous pretreatment composition ofclaim 1, further comprising hydrogen peroxide.
 5. The aqueouspretreatment composition of claim 1, wherein the trivalent chromiumcompound is present in an amount of between about 10% wt. % to about0.5% wt. %.
 6. The aqueous pretreatment composition of claim 1, whereinx is 1.5.
 7. The aqueous pretreatment composition of claim 1, wherein xis
 2. 8. An aqueous pretreatment composition for treating a metalsurface, the composition consisting of: water; silica; a trivalentchromium compound with the formula Cr(H_(x)PO₄)₃, where x can be 1.5 or2.
 9. The aqueous pretreatment composition of claim 8, wherein thepretreatment composition is acidic.
 10. The aqueous pretreatmentcomposition of claim 8, wherein the pretreatment composition has a pH ofbetween about 1 and about
 4. 11. The aqueous pretreatment composition ofclaim 8, wherein the pretreatment composition is acidic.
 12. The aqueouspretreatment composition of claim 8, wherein the trivalent chromiumcompound is present in an amount of between about 10% wt. % to about 5%wt. %.
 13. The aqueous pretreatment composition of claim 8, wherein x is1.5.
 14. The aqueous pretreatment composition of claim 8, wherein x is2.
 15. A method for preparing a metal pretreatment compositionconsisting of: reducing a hexavalent chromium compound, with hydrogenperoxide, in the presence of phosphoric acid to create a trivalentchromium compound with the formula Cr(H_(x)PO₄)₃, where x can be 1.5 or2, at a pH of between about 1 and about 4; and adding silica after thehexavalent chromium compound has been fully reduced.
 16. The method ofclaim 15, wherein the silica is Aerodisp W7512S.
 17. A method forcoating a metal surface comprising: roll coating the metal surface withthe composition of claim 1; spraying the metal surface for a period ofup to about 300 seconds in the composition of claim 1; or dipping themetal surface for a period of up to about 300 seconds with thecomposition of claim
 1. 18. The method of claim 17 wherein the metalsurface is selected from the group consisting of zinc, zinc alloys,aluminum, aluminum alloys, steel, zinc coated steel, and zinc withaluminum alloy.